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Tear down of Massdrop THX AAA 789 Headphone Amp

amirm

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#1
With kind permission of the owner, I took apart the Massdrop THX AAA 789 headphone amplifier. If you have not seen it yet, check my my review of Massdrop THX 789.

The internal structure of this unit follows other models from Massdrop. I was impressed with the effort to keep all the components on one side using mostly SMT (surface mount) parts. The bottom of the PCB is only through-hole pins which I am assuming was wave soldered.

Here is lower resolution shot of the PCB with my annotations:

Massdrop THX AAA 789 teardown main PCB reduced2.jpg


Click on this attachment to get 4X higher resolution image.

Massdrop THX AAA 789 teardown main PCB reduced.jpg

I have 4X larger image yet again. Send me a message if you want that.

The biggest shock here is that there is no "THX module" of any sort! No custom or hybrid parts that I can see. Instead, we have a bunch of exclusively Texas Instruments parts. I have what I think is the sum total of it encased in white square, with a set of OPA 1602A op-amps providing the feed forward/low power output, augmented by pair of high power OPA 564 amps to drive larger signals.

With no custom or proprietary parts, the design is open to replication by Asian vendors.

The auto-off and power signals are in the top left part which I have zoomed in and shown beside it. There is an audio signal detector from JRC that allows them to automatically power the unit on and off based on whether there is music signal or not. And that dreaded logic for power on/off switch.

Fair number of relays are on the board. Interesting to see a pair of them by the RCA input signals. Do they shut them off when balanced input is in use?

The input stage is made up of a bunch of OPA 1602A op-amps. Curious to see room for two more relays there but only one populated. Likewise, there is room for two large electrolytic caps in the middle right that are missing. I don't see the need for them give total absence of any power supply noise but maybe you want to add them to make yourself feel better post warranty. :)

I see no hand modifications or soldering of any sort. This may have been behind the delays, with them turning the PCB multiple times to make sure it is fully automated assembly with no need for manual labor to fix things.

Here the links to various ICs used:
OPA564: http://www.ti.com/lit/ds/symlink/opa564.pdf

1542692074496.png


TL062: http://www.ti.com/lit/ds/symlink/tl062.pdf

1542692151631.png


OPA1602: http://www.ti.com/lit/ds/symlink/opa1604.pdf

1542692196351.png


OPA564: http://www.ti.com/lit/ds/symlink/opa564.pdf

1542692293441.png


JRC NJU7181: https://www.njr.com/semicon/PDF/NJU7181_E.pdf
1542692357995.png


TI LM239: http://www.ti.com/lit/ds/symlink/lm239.pdf
1542692462594.png


Conclusions
The Massdrop THX AAA 789 has a rather high quality and optimized mechanical and electronic design. Nothing jumps out at me to note as problem areas.

The big surprise is lack of any custom parts to implement the THX feed forward amplification logic. Instead, this is THX know-how, and design of a discrete amplifier using standard off-the-shelf ICs and parts. Even though they say they have patent protection, they have left themselves open to reverse engineering by the usual people. The core ideas behind feed forward design are 20+ years old anyway so any patents on the main idea have long expired. Not encouraging anyone to go and copy their design of course. They are not charging hardly any premium here so might as well get the real deal.

I hope the other experts on the forum study the details and chime in with what they figure out. I just performed a cursory look and have made little attempt at fully documenting the unit.

Overall, a thumbs up for Massdrop THX AAA 789 from build quality.

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rajapruk

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#4
Thanks!

Do you see any dc-protection caps on the input? Might it be C12 and C13 that eventually got emitted at some later stage of the product development?
@AndrewMason said in a Massdrop comment a long time ago there would be dc-protection caps on the input that could be shorted/bypassed, and then maybe giving even lower LF distortion. Warranty-voiding of course.

Interesting to see how tight all SMD-components sits together. This is probably one of the keys to get this level of performance, avoiding unwanted parasitic capacitance etc.

Regarding reverse engineering, is that easy to do on a pcb like this with several layers? I'm just guessing we have 4 layers, or more, of connections. But maybe it is enough to see which SMD-components are used to copy it conceptually?

I am glad there are so few electrolytic capacitators in there that can go bad/old. Looks like this one will live forever :)
 
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amirm

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#5

amirm

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#7
so much space, maybe I can integrate khadas tone board into it.....
that will be a great DAC/AMP.....
Interesting idea. They could have had a header for such a board to plug-in. Otherwise, one can easily be mounted there.
 

rajapruk

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#11
I see no hand modifications or soldering of any sort. This may have been behind the delays, with them turning the PCB multiple times to make sure it is fully automated assembly with no need for manual labor to fix things.
What delays? They were all shipped to customer before the originally stated shipping date. That is the opposite of a delay :)
 

pos

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#12
Many thanks to @amirm and @Sythrix for this teardown!

The power switch logic section seems quite complicated. I would have preferred a simple on/off switch: the auto off function is nice but without auto on it is not really useful IMHO.
@amirm, do you think this could be modified to automatically power on when main power is switched on? Or maybe using an external trigger?
 

Veri

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#13
Pretty surprised by the discrete nature of the amp but very clean looking none the less!
 

rajapruk

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#14
Many thanks to @amirm and @Sythrix for this teardown!

The power switch logic section seems quite complicated. I would have preferred a simple on/off switch: the auto off function is nice but without auto on it is not really useful IMHO.
@amirm, do you think this could be modified to automatically power on when main power is switched on? Or maybe using an external trigger?
Don’t you think this logic steers a lot of relays etc, giving you the much sought after totally pop free operation? Maybe this is hard to acheive with a simple on/off?
Btw, did you try to cut the power to it, does it pop then?
 

LeoL

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#16
I may be wrong, but the trio of op amps at the bottom left of the pic look to be summing the differential input to single ended before the volume pot (which looks to be of the 2-channel variety); after the pot, the signal looks like it’s getting converted back to differential again at the “THX” section with the quad output buffers driving the balanced phone jack. For the purists here, no, not “fully balanced” all the way through, then. Not that it really matters, given the wonderful measurements on Amir’s test bench.
 

Sythrix

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#19
For the purists here, no, not “fully balanced” all the way through, then. Not that it really matters, given the wonderful measurements on Amir’s test bench.
Though it would be interesting to see a comparison of the same technology in a fully balanced configuration, as I believe the Benchmark THX is. I'm pretty sure Amir said he would be receiving and reviewing that one as well.
 

andymok

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#20
I may be wrong, but the trio of op amps at the bottom left of the pic look to be summing the differential input to single ended before the volume pot (which looks to be of the 2-channel variety); after the pot, the signal looks like it’s getting converted back to differential again at the “THX” section with the quad output buffers driving the balanced phone jack. For the purists here, no, not “fully balanced” all the way through, then. Not that it really matters, given the wonderful measurements on Amir’s test bench.
I guess it's actually easier/better to do attenuation in single-ended?
 
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